原始问题转换为对偶最优化问题

原始问题:

minw,b,ξ12w2+Ci=1Nξi1s.t.yi(wxi+b)1ξiξi0,i=1,2,,N\begin{aligned} \min_{w,b,\xi} \quad \frac{1}{2}||w||^2 + C \sum_{i=1}^N\xi_i && {1} \\ s.t. \quad y_i(w\cdot x_i+b)\ge 1-\xi_i \\ \quad \quad \xi_i \ge 0, i = 1,2,\cdots,N \end{aligned}

对偶问题:

mina12i=1Nj=1Naiajyiyj(xixj)i=1Nais.t.i=1Naiyi=00aiC,i=1,2,,N2\begin{aligned} \min_a \quad \frac{1}{2}\sum_{i=1}^N\sum_{j=1}^Na_ia_jy_iy_j(x_i\cdot x_j) - \sum_{i=1}^Na_i \\ s.t. \quad \sum_{i=1}^Na_iy_i=0 \\ \quad \quad 0 \le a_i \le C, i=1,2,\cdots,N && {2} \end{aligned}

原始问题公式(1)到对偶问题公式(2)的推导过程

  1. 将原始问题(1)稍加变形

    minw,b,ξ12w2+Ci=1Nξi3s.t.1ξiyi(wxi+b)0ξi0,i=1,2,,N\begin{aligned} \min_{w,b,\xi} \quad \frac{1}{2}||w||^2 + C \sum_{i=1}^N\xi_i && {3} \\ s.t. \quad 1-\xi_i - y_i(w\cdot x_i+b) \le 0 \\ \quad \quad -\xi_i \le 0, i = 1,2,\cdots,N \end{aligned}
  2. 写出拉格朗日函数

    L(w,b,ξ,a,μ)=12w2+Ci=1Nξi+i=1Nai(1ξiyi(wxi+b))i=1Nμiξi4\begin{aligned} L(w, b, \xi, a, \mu) = \frac{1}{2}||w||^2 + C \sum_{i=1}^N\xi_i + \sum_{i=1}^Na_i(1-\xi_i - y_i(w\cdot x_i+b)) - \sum_{i=1}^N\mu_i\xi_i && {4} \end{aligned}
  3. L(w,b,ξ,a,μ)L(w, b, \xi, a, \mu)分别对w,b,ξiw, b, \xi_i求偏导,并令导数为0

    wL(w,b,ξ,a,μ)=wi=1Naiyixi=0bL(w,b,ξ,a,μ)=i=1Naiyi=0ξiL(w,b,ξ,a,μ)=Caiμi=05\begin{aligned} \nabla_wL(w, b, \xi, a, \mu) = w - \sum_{i=1}^Na_iy_ix_i = 0 \\ \nabla_bL(w, b, \xi, a, \mu) = -\sum_{i=1}^Na_iy_i = 0 \\ \nabla_{\xi_i}L(w, b, \xi, a, \mu) = C - a_i - \mu_i = 0 && {5} \end{aligned}
  4. 将等式(5)代入公式(4)得到对偶函数:

    L(w,b,ξ,a,μ)=12w2+Ci=1Nξi+i=1Nai(1ξiyi(wxi+b))i=1Nμiξi=12(ww)+i=1N(Caiμi)ξi+i=1Naiwwbi=1Naiyi=12(ww)+i=1Nai=12i=1Nj=1Naiajyiyj(xixj)i=1Nai6\begin{aligned} L(w, b, \xi, a, \mu) = \frac{1}{2}||w||^2 + C \sum_{i=1}^N\xi_i + \sum_{i=1}^Na_i(1-\xi_i - y_i(w\cdot x_i+b)) - \sum_{i=1}^N\mu_i\xi_i \\ = \frac{1}{2}(w \cdot w) + \sum_{i=1}^N(C-a_i-\mu_i)\xi_i + \sum_{i=1}^Na_i - w \cdot w - b\sum_{i=1}^Na_iy_i \\ = -\frac{1}{2}(w \cdot w) + \sum_{i=1}^Na_i \\ = \frac{1}{2}\sum_{i=1}^N\sum_{j=1}^Na_ia_jy_iy_j(x_i\cdot x_j) - \sum_{i=1}^Na_i && {6} \end{aligned}
  5. 公式(6)为对偶函数 公式(5)能得出以下限制条件:

    i=1Naiyi=00aiC,i=1,2,,N7\begin{aligned} \sum_{i=1}^Na_iy_i=0 \\ 0 \le a_i \le C, i=1,2,\cdots,N && {7} \end{aligned}
  6. 公式(6)结合公式(7)就是原始问题的对偶问题

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